This invention relates to fluorinated curable compositions of the addition curing type having high thixotropy, curing into gel products having improved chemical resistance and solvent resistance, and thus suitable for use as potting, sealing and coating materials for electric and electronic parts, and a variety of articles sealed with the cured compositions.
Cured gel products of silicone rubber have good electrically and thermally insulating properties, stable electric properties and flexibility. They are useful as potting and sealing materials for electric and electronic parts, and coating materials for protecting control circuit devices such as power transistors, ICs and capacitors from external, thermal and mechanical damages.
Typical silicone compositions capable of forming such cured gel products are organopolysiloxane compositions of the addition curing type. The organopolysiloxane compositions of the addition curing type are known, for example, from JP-A 56-143241, 62-3959, 63-35655 and 63-33475 as comprising an organopolysiloxane having vinyl groups attached to silicon atoms and an organohydrogenpolysiloxane having hydrogen atoms attached to silicon atoms, wherein crosslinking reaction takes place in the presence of a platinum group catalyst to form a silicone gel.
Also known in the art are fluorosilicone gel compositions comprising an organopolysiloxane having trifluoropropyl groups (see JP-A 7-324165).
However, these organopolysiloxane compositions of the addition curing type result in silicone gels which are prone to degradation or swelling upon exposure to such chemicals as strong bases and strong acids or such solvents as toluene, alcohols and gasoline, often failing to maintain their performance.
As one solution to this problem, JP-A 11-116685 discloses a fluorochemical gel composition comprising a polyfluoro compound having two alkenyl groups and a divalent perfluoropolyether group in a molecule, an organohydrogen-polysiloxane having hydrogen atoms attached to silicon atoms, and a platinum group catalyst as well as a fluorochemical gel product resulting from curing thereof.
When gel products are used for protection purpose, gel compositions having a gel-forming ability must be applied onto surfaces of electronic parts by spot potting. However, prior art gel compositions are not amenable to spot potting on account of fast flow, failing to form an effective protective layer on electronic parts. Therefore, there is a need for a gel composition which is reasonably flowable and resistant to chemicals and solvents.
The fluidity of gel compositions can be reduced by increasing their viscosity. However, the compositions lower their viscosity upon heating for curing, yet failing to form an effective protective layer on electronic parts.
Therefore, an object of the invention is to provide a fluorinated curable composition which can form a flexible cured gel product and is so thixotropic that it is amenable to potting due to a low apparent viscosity when shear stresses are applied during potting operation by means of a dispenser or the like, but it does not substantially flow due to a high apparent viscosity when no shear stresses are applied. Another object of the invention is to provide an article, such as an electric or electronic part, sealed with the cured product of the composition.
The invention provides a thixotropic fluorinated curable composition comprising
(A) a linear polyfluoro compound of the general formula (1):
CH2xe2x95x90CHxe2x80x94(X)axe2x80x94Rf1xe2x80x94(Xxe2x80x2)axe2x80x94CHxe2x95x90CH2xe2x80x83xe2x80x83(1)
xe2x80x83wherein X is xe2x80x94CH2xe2x80x94, xe2x80x94CH2Oxe2x80x94, xe2x80x94CH2OCH2xe2x80x94 or xe2x80x94Yxe2x80x94NR1xe2x80x94COxe2x80x94, Y is xe2x80x94CH2xe2x80x94 or a group of the following structural formula (Z): 
xe2x80x83Xxe2x80x2 is xe2x80x94CH2xe2x80x94, xe2x80x94OCH2xe2x80x94, xe2x80x94CH2OCH2xe2x80x94 or xe2x80x94COxe2x80x94NR1xe2x80x94Yxe2x80x2xe2x80x94, Yxe2x80x2 is xe2x80x94CH2xe2x80x94 or a group of the following structural formula (Zxe2x80x2): 
xe2x80x83R1 is hydrogen, methyl, phenyl or allyl, Rf1 is a divalent perfluoropolyether group, and xe2x80x9caxe2x80x9d is independently 0 or 1,
(B) an organohydrogensiloxane having at least two hydrogen atoms each attached to a silicon atom in a molecule,
(C) an effective amount of a platinum group metal catalyst,
(D) finely divided silica hydrophobized with a polysiloxane and having a specific surface area of at least 50 m2/g, and optionally,
(E) a second linear polyfluoro compound of the following general formula (2):
Rf2xe2x80x94(X)axe2x80x94CHxe2x95x90CH2xe2x80x83xe2x80x83(2)
xe2x80x83wherein X and xe2x80x9caxe2x80x9d are as defined above, and Rf2 is a monovalent perfluoropolyether group.
Also contemplated herein are an electric or electronic part, pressure sensor, rotation sensor and air flow meter which is sealed with the thixotropic fluorinated curable composition in the cured state.
It has been found that the above and other objects are effectively achieved by combining components (A) through (D) or components (A) through (E), and especially by compounding silica (D) in an amount of 0.5 to 8 parts by weight per 100 parts by weight of component (A) or components (A) and (E) combined, because the resulting composition is satisfactorily thixotropic and capable of forming a flexible cured gel product.